Applicability of a high energy plasma to a number of key semiconductor fabrication processes has made it a virtually indispensable production technology. An essential step in developing microwave plasma processing is the electron cyclotron resonance discharge where, as the name of the resonance implies, the microwave field oscillates in resonance with the electron cyclotron motion about the magnetic field lines. In other words, where microwave energy is coupled to the natural resonant frequency of the electron gas in the presence of a static magnetic field and when the electron cyclotron frequency and the excitation frequency are equal to each other, the condition for the resonance happens. In practice, this requirement can be met, for example in a volume within the discharge, with the static magnetic field made correspondent to resonance, that is where the both frequencies coincide with each other, and a component of electric field is normal to the static magnetic field. The electrons are accelerated in this electron cyclotron resonance volume ionizing and exciting the neutral gas. As a result, a plasma comes into existence varying from a weakly to a highly ionized state depending on discharge pressure, gas flow rate, and input microwave power. The power absorption is localized to the resonance zone in an electron cyclotron resonance source resulting in very precise control on the power deposition profile.
In a conventional electron cyclotron resonance discharge, the plasma region produced is not extendible over a large area, and is generally limited to a diameter of approximately 8.about.18 cm. It is mainly limited by the use of waveguide as the applicator while the waveguide operates in the fundamental mode.
Various efforts have been mounted in making larger area plasma. There are at least three types of designs that seem to outperform the others. The type described in J. Asmussen. "Electron Cyclotron Resonance Microwave Discharges For Etching And Thin-film Deposition." J. Vac. Sci. Technol. A7(3), May/June 1989, 883-893, utilizes a partial area of a large cavity to produce plasma while the remaining area is used for mode stabilization. The type described in G. Neumann, et al. "Characterization Of A New Electron Cyclotron Resonance Source Working With Permanent Magnets." J. Vac. Sci. Technol. B9(2), March/April 1991, 334-338, harnesses a horn antenna as the applicator which operates in the TM.sub.01 mode. The diameter of the plasma can reach 20 cm. Even though this is an improved design, however, it has reached its upper limit in the plasma diameter and the area can no longer be increased. The third type which is described in R. R. Burke et al. "Distributed Electron Cyclotron Resonance in Silicon Processing: Epitaxy And Etching." J. Vac. Sci. Technol. A8(3), May/June 1990, 2931-2938, requires difficult impedance matching and uneasy applicator design. The processing is complicated when the antenna is used inside the plasma.
It has ben generally observed that in attempts to extend microwave plasma over larger areas, an unstable mode is easily produced resulting in a mode conversion loss and uneven distribution of the plasma.
It is therefore an object of the present invention to provide a large cavity waveguide that does not have the drawbacks of the prior art waveguides.
It is another object of the present invention to provide a large cavity waveguide that has a corrugated interior construction.
It is still another object of the present invention to provide a large cavity waveguide that can be operated at a stable microwave mode.
It is yet another object of the present invention to provide a large cavity waveguide that has the generated plasma extended over an area larger than the prior art waveguides.